Modified Microgrinding Process

ABSTRACT

A method of forming a substrate is performed by grinding a substrate using abrasives so that both major surfaces of the substrate achieve desired flatness, smoothness, or both. In an embodiment, a coarser abrasive is used to grind one major surface, while a finer abrasive is simultaneously used to grind the other major surface. A single grinding step can used to produce a substrate having opposing surfaces of different surface roughnesses. This may help to eliminate a typical second downstream fine polishing step used in the prior art. Embodiments can be used with a wide variety of substrates, including sapphire, silicon carbide and gallium nitride single crystal structures grown by various techniques.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) to U.S. Patent Application No. 61/707,528 entitled “Modified Grinding Process,” by Rizzuto et al., filed Sep. 28, 2012, which is assigned to the current assignee hereof and incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

This disclosure, in general, relates to semiconductor substrates and in particular to sapphire substrates and methods of manufacturing such substrates.

BACKGROUND

In many types of manufacturing, including for example manufacturing of sapphire substrates for use in LED manufacturing, it is common to grind, lap, or polish a substrate so that both major surfaces (faces) meet certain minimum levels of flatness, smoothness, or both. In general, grinding can be defined as rapid material removal either to reduce it to a suitable size or to remove large irregularities from the surface through the use of a relatively coarse abrasive (>40 μm), typically in the form of an abrasive pad or disk. The term “lapping” is usually used to refer to removal of material using free abrasive particles such as an abrasive slurry. Finally, polishing is the removal of material to produce a scratch-free, mirror-like surface using fine (<3 microns) abrasive particles. All of these material removal processes can make use of a variety of abrasive materials such as abrasive slurries or fixed abrasive pads or disks, and in practice the lines between the different categories tend to blur. Collectively, all of these processes can be generally referred to herein as “abrasive processes.”

One example of a wafer or substrate processing tool is the typical double-sided lapping machine 100, schematically illustrated in FIG. 1. Such a tool can include two superposed platens or lapping plates 102 respectively disposed over and under a substrate 104, so that opposing surfaces of the substrate can be processed simultaneously. An abrasive slurry, typically containing abrasive particles 106 in the 5 micron to 180 micron range, is applied directly to the lapping plates. As shown in FIG. 2, the double-sided lapping machine 100 includes a plurality of carriers 202, with each carrier holding a plurality of substrates or wafers 204. Each lapping plate may have an internal ring gear 206 around the outer periphery of the plate and an inner central gear 208. Each of the carriers can also have a toothed outer periphery 210, which engages with the inner and outer gears. Rotation of the inner gears (as shown by arrow 212) and outer gears (as shown by arrow 214) in opposite directions causes each of the carriers to rotate both about the axis of each carrier (as shown by arrow 216) and around the axis of the lapping plate (as shown by arrow 218). The resulting relative movement between the rotating carriers and lapping plates forms a cycloidal curve, similar to the movement of planets as they rotate on their own axes while simultaneously orbiting a sun. This rotation in the presence of the abrasive slurry abrades away material on both major surfaces of the substrates.

Single-sided lapping machines are also known, but these machines only process one side of the substrates at a time. Also, similar planetary double-sided grinding machines are sometimes used to remove material from wafers or substrates using various types of fixed abrasives or pads.

Typically, the process of manufacturing a suitably flat substrate, such as a silicon wafer or a sapphire wafer, includes a number of grinding, lapping, or polishing steps, no matter what type of process or abrasive material is used. For example, when double-sided grinding machines are used, the substrate is initially processed using a coarse fixed abrasive to quickly remove excess material and the worst of the surface damage cause by sawing the substrates from a boule. Depending on the application, the first coarse grinding step can be followed by one or more fine grinding steps to produce a suitably smooth and flat surface. Fine grinding can be followed by a polishing step to produce a very smooth mirror surface on the substrate. Often the smoothest surface is only needed on one side of the substrate. A relatively coarse surface would be acceptable, or even desirable, on the other surface. However, the progressively finer grinding steps are needed to prepare the one surface for polishing. Because the various grinding steps are done sequentially, when double-sided grinding is being used it is common to apply the fine grinding steps to both sides anyway. This results in unnecessary expense, both in terms of time and additional supplies and equipment wear.

As such, an improved method of substrate material removal would be desirable.

SUMMARY

Embodiments described herein are applicable to the preparation (manufacture) of any hard substrates, such as oriented single crystal substrates, by grinding a substrate wafer using abrasives so that both major surfaces of the substrate meet certain minimum levels of flatness, smoothness, or both. In particular embodiments, a coarser abrasive is used to grind one major surface of the wafer, while a finer abrasive is simultaneously used to grind the other major surface of the wafer. As a result, a single grinding step can produce a wafer having opposing surfaces of a different surface roughness. This allows the coarser abrasive to be used for preferential material removal to thin the wafer, while the fine abrasive produces a surface smooth enough for many uses or for further polishing—thus eliminating or reducing the demand for the typical second downstream fine polishing step used in the prior art. Particular embodiments can be used with a wide variety of substrates, including sapphire, silicon carbide and gallium nitride single crystal structures grown by various techniques.

The foregoing has outlined rather broadly the features and technical advantages of particular embodiments in order that the detailed description that follows may be better understood. Additional features and advantages of embodiments will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of embodiments as described herein. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings.

FIG. 1 is a schematic diagram of a prior art double-sided lapping machine.

FIG. 2 is a schematic diagram showing the lower lapping plate and the wafer carriers of the double-sided lapping machine of FIG. 1.

FIG. 3 is a cross-sectional view of a double-sided grinding machine in accordance with a particular embodiment.

FIG. 4 is a schematic diagram of a grinding fluid filtration system in accordance with a particular embodiment.

FIG. 5 is a photograph of a double-sided grinding machine suitable for practicing embodiments.

The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing.

DESCRIPTION OF THE DRAWINGS

This invention is applicable to the preparation (manufacture) of substrates, such as oriented single crystal substrates, by grinding and polishing a substrate wafer using fixed abrasives so that both major surfaces of the substrate meet certain minimum levels of flatness, smoothness, or both. Particular embodiments as described herein can be used with a wide variety of substrates, including sapphire, silicon carbide and gallium nitride single crystal structures grown by various techniques.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples are illustrative only and not intended to be limiting. To the extent not described herein, many details regarding specific materials and processing acts are conventional and may be found in textbooks and other sources within the crystal formation and processing arts.

Abrasives can be generally categorized as free or loose abrasives and fixed abrasives. Loose abrasives are generally composed of abrasive grains or grits in powder or particulate form in a liquid medium that forms a suspension, commonly known as a slurry. Fixed abrasives utilize abrasive grits within a matrix of material which fixes the position of the abrasive grits relative to each other. Fixed abrasives generally include coated abrasives, like sandpaper, bonded abrasives, or the like. In bonded abrasives, the abrasive grits are fixed in position relative to each other by use of a matrix material, wherein the grits are distributed. Particular embodiments described herein utilize fixed abrasive components in the form of coated or bonded abrasives. Loose abrasive lapping and fixed abrasive “microgrinding” (also referred to as grinding with lapping kinematics, grinding with planetary kinematics, and fixed abrasive grinding) are operations used in the batch processing of single and polycrystalline materials such as sapphire and silicon carbide, ceramics, glasses, metallic components, etc.

Typically, to form substrates suitable for use as a substrate for semiconducting devices, particularly, light emitting diodes/laser diodes (LED/LD) applications, the process begins with a bulk material out of which the final processed substrates will be formed. One prior art method of forming sapphire substrates suitable for LED/LD applications is described in U.S. Pat. No. 8,197,303 to Tanikella et al. for “Sapphire substrates and methods of making same,” which is incorporated herein by reference in its entirety and which is assigned to the assignee hereof.

For sapphire substrates, the process can be initiated by forming a boule or a ribbon of single crystal sapphire. As will be appreciated, the sapphire can be formed into a blank, a boule, or a ribbon having any size or shape suitable for use as a substrate for semiconducting devices, particularly, LED/LD applications. As such, a common shape is a boule having a substantially cylindrical contour. For a ribbon, a common shape is a sheet. The formation of single crystal sapphire can be accomplished using techniques such as the Czochralski Method, Edge-Defined Film Fed Growth (EFG), or Kyropoulos Method, or other techniques depending upon the desired size and shape of the boule or ribbon, and the orientation of the crystal.

After forming the single crystal sapphire, sawing of the boule or blank can be undertaken to section the sapphire and form wafers. Wire sawing of the sapphire boule provides a plurality of unfinished sapphire wafers. Generally, the duration of the wire sawing process can vary from about a few hours, such as about 2.0 hours to about 30 hours. In general, the desired thickness of the unfinished sapphire is from 1.0 to 10 mm. The wire sawing can be carried out by using a fixed abrasive wire element or elements, such as an array of wires plated or coated with abrasive grains. One example of this technology is non-spooling type wire sawing such as FAST (fixed abrasive slicing technology), offered by Crystal Systems Inc. of Salem, Mass. Another example is spool-to-spool wire sawing systems. In the case of single crystal raw stock produced by the EFG process, typically in the shape of a ribbon or sheet, the wire sawing process may not be necessary, and cored-out (shaped) wafers can proceed directly to a grinding step.

After forming a plurality of sapphire wafers via sawing, the surfaces of the unfinished sapphire wafers can be processed. Typically, both major opposing surfaces of the unfinished sapphire wafers will undergo grinding or lapping to improve the finish of the surfaces. Conventional coarse abrasive processes include abrading both major surfaces of the unfinished sapphire substrates, for example, using double-sided grinding or lapping. Generally, the coarse abrasive process removes a sufficient amount of material to remove major surface irregularities caused by the wire sawing process, at a reasonably high material removal rate. As such, the coarse abrasive process typically removes at least 30 to 50 microns of material from the major surfaces (faces) of the unfinished sapphire wafers.

Where fixed abrasives are used, the coarse abrasive grains can include conventional abrasive grains such as crystalline materials or ceramic materials including alumina, silica, silicon carbide, zirconia-alumina, another suitable abrasive, or any combination thereof. In addition to or alternatively, the coarse abrasive grains can include super-abrasive grains, including diamond, cubic boron nitride, or mixtures thereof. The coarse abrasive grains can have a mean particle size, for example, of 60 to 300 microns. For bonded adhesives, the bond material matrix can include a metal or metal alloy. A suitable metal includes iron, aluminum, titanium, bronze, nickel, silver, zirconium, alloys thereof, or the like. Examples of particular abrasive wheels include those described in U.S. Pat. No. 6,102,789; U.S. Pat. No. 6,093,092; and U.S. Pat. No. 6,019,668, incorporated herein by reference in their entireties.

The typical coarse grinding process includes providing an unfinished sapphire wafer on a holder and rotating the sapphire wafer relative to a coarse abrasive surface. A double-sided grinding machine, similar to the double sided lapping machine shown in FIGS. 1-2, can be used. By way of example, the grinding plates can be rotated at speed of 60 to 500 rpm. Typically a liquid coolant or grinding fluid is also used. After coarse grinding, sapphire wafers typically have an average surface roughness R_(a) of 0.2 to 1 microns.

Once the coarse grinding has been completed, the sapphire wafers can be subject to a fine grinding process to produce a smoother surface. This fine grinding step removes less material from the surface of the substrate, usually 10 to 15 microns.

The fine abrasive particles can be of the same general materials as the coarse abrasives and can use the same types of bonding materials. The difference, of course, is that the fine abrasive particles are smaller than the coarse abrasives. For example, the fine adhesive particles can have a mean particle size of 2 to 75 microns. Generally, the difference in mean particle sizes between the coarse and fine fixed abrasives is at least 20 microns.

A double-sided grinding machine, similar to the one shown in FIGS. 1-2, can also be used for fine grinding or polishing using bonded abrasives. By way of example, the grinding plates can be rotated at speed of 60 to 1000 rpm. Typically a liquid coolant or grinding fluid is also used. After fine grinding, sapphire wafers typically have an average surface roughness R_(a) of about 0.10 microns to 1.0 microns.

After fine grinding, the sapphire wafers can be subjected to a stress relief process such as those disclosed in EP 0 221 454 B1. As described, stress relief may be carried out by an etching or annealing process. Annealing can be carried out at a temperature above 1000° C. for several hours.

After the fine grinding step, the sapphire wafers can be subjected to a polishing step to produce an even smoother surface. This polishing step removes even less material from the surface of the substrate, usually 1 micron to 4 microns. This polishing step generally makes use of an abrasive slurry having abrasive particles with an average particle diameter of less than 1 micron, typically less than 200 nanometers. A particularly useful loose abrasive for such a polishing process is alumina, such as in the form of polycrystalline or monocrystalline gamma alumina.

Typically, polishing is undertaken on only one surface, as opposed to the grinding steps described above, which usually includes grinding both major surfaces of the unfinished sapphire wafers. After polishing, sapphire wafers typically have an average surface roughness R_(a) of approximately 10 to 400 angstroms (0.001 micron to 0.04 micron).

Significantly, in the prior art, at least two separate grinding steps (one coarse and one fine) have been required before final polishing. For many applications, however, only a coarse grinding step would be required on one surface, while the other surface requires at least one additional fine grinding step followed by polishing. In the prior art, substrate processing operations are designed so that the abrasive processes performed on the top side of the component are the same as those performed on the bottom side. Hence, the final surface finish or texture on the top and bottom sides is the same (before a final polishing step, if any). Some substrates, such as C-plane sapphire or single crystal SiC used in LED manufacturing, require a subsequent polishing step to improve the surface quality on only one side of the wafer.

Particular embodiments as described herein can make use of a novel microgrinding process as a replacement to the conventional lapping processes for any hard substrate, such as the finishing of oriented single crystal bodies. In microgrinding, the abrasive slurry used in lapping is replaced with a fixed abrasive product. Microgrinding using bonded fixed abrasives provides a number of advantages over the use of abrasive slurries, most notably that the material removal rates can be substantially increased by applying a higher load (pressure) between the abrasive and the substrate. The use of fixed abrasives disposed over the working surfaces of the grinding plates instead of abrasive slurries also reduce maintenance costs and the accompanying unproductive time associated with periodic dressing of the plates to the necessary degree of flatness and coplanarity. Microgrinding using bonded fixed abrasives will also produce less sub-surface damage (when operating parameters are optimized) than lapping using an abrasive slurry.

In accordance with particular embodiments described herein, the design of at least one of the fixed abrasive plates or wheels used in the microgrinding process is modified to generate opposing substrate surfaces of differing quality (finish, sub-surface damage, texture, etc.), such that the need for the second downstream fine abrasive process can be eliminated or reduced. In some instances, the need for a final polishing step can also be eliminated or at least greatly reduced. In the case of a sapphire wafer that will act as the substrate in LED production, for instance, the top plate or wheel used in the microgrinding process preferably uses a finer abrasive grit than the bottom plate so that the desired surface finish on each side of the wafer is achieved, or nearly achieved, on a single operation. In some applications, downstream fine abrasive processes will still be required, but the need for such fine abrasive processes can be substantially reduced, which is significant because such processes are both time-consuming and expensive.

Particular embodiments as described herein make use of double-side grinding with planetary kinematics using a grinding machine that is very similar to the double-sided lapping machine of FIGS. 1 and 2. The abrasive slurry used commonly used in lapping is replaced with two fixed bonded abrasive plates or wheels, which are mounted onto the upper and lower coaxial grinding plates. The abrasive particles in the bonded abrasive plates can include diamond, cubic boron nitride, silicon carbide, alumina, zirconia, another suitable abrasive material, or any combination thereof. The abrasive particles can also be of various regular or irregular shapes (circular, square, hexagonal, etc.) and size. These abrasive particles are bonded together in a resin, vitreous or metal matrix to form the rigid substrate or plate used for the microgrinding process.

FIG. 3 is a cross-sectional view of a double-sided grinding machine 300 in accordance with a particular embodiment. As in the prior art lapping process described above, substrates 304 to be processed are preferably held in a carrier 301 that is disposed between two grinding plates 302, 303 on which the fixed abrasive plates 308, 310 are mounted. The grinding plates are brought together to exert a predetermined pressure upon the substrates while the plates, carriers, substrates, or any combination thereof are rotated, thus planarizing, polishing, thinning, or a combination thereof the surfaces of the substrates. Preferably, the two grinding plates each have a fixed abrasive having different sized abrasive particles. In other words, one grinding plate has an abrasive that is coarser than the abrasive of the other grinding plate. As discussed in greater detail below, the coarser grit abrasive plate 310 may be on the lower or bottom grinding plate 303, while the finer grit abrasive plate 308 is on the upper or top grinding plate 302. In various embodiments, the abrasive-containing grinding plates can rotate in the same direction or opposite directions. One plate can also be held in a fixed position, while the other plate is rotated. In this manner, one surface of the substrate can be processed to a smoother surface than the opposite surface, and at least a portion of the grinding of both surfaces can occur simultaneously.

FIG. 5 is a photograph of a double-sided grinding machine suitable for practicing embodiments as described herein.

In accordance with particular embodiments, in the case when different roughness (texture) is desired between the top and bottom surfaces of the substrate to be processed, one grinding plate can use a fine-grit abrasive product while the other plate can use a coarser grit abrasive product. For instance, the top plate could be made with a fine-grit abrasive product to generate a surface with very low roughness, thus reducing or eliminating any downstream polishing process time required to achieve the final surface characteristics. The bottom plate could contain a coarser-grit abrasive product to generate a surface more typical of lapping or grinding operations. The grit size chosen is preferably dictated by the desired roughness/texture of the side to be processed and by the amount of substrate material to be removed.

By using two fixed abrasive plates having different grit size, the coarser grit abrasive can be used for the majority of material removal, for example if it is desirable to thin the substrate. The surface left by the coarser abrasive will be rougher than the surface produced by the finer grit, but in many cases that is either not significant or actually desirable. For example, a polished upper surface in the case of sapphire wafers is advantageous for promoting the growth of compound thin semiconductor films and devices, whereas a rougher bottom surface is thought to promote heat transfer.

The finer grit abrasive plate will not remove as much material but will produce a smoother surface, ready for any additional polishing step. Significantly, because both sides of the substrate can be processed simultaneously by the abrasive plates having different grit sizes, the process of producing the desired substrate is much faster than the prior art, which required multiple sequential grinding steps.

Even in more demanding applications where one or more polishing steps will still be required, the elimination of sequential separate coarse and fine abrasive process steps saves a significant amount of time. Each abrasive process step typically requires 15-30 minutes to complete, and often requires moving the substrates to a completely different grinding machine for each grinding step. By grinding the top surface using a fine abrasive at the same time the bottom surface is processed with a coarser abrasive, steps in the process are eliminated, less equipment is required, and supply costs are reduced (since no abrasives are wasted smoothing the bottom surface more than is required).

As will be recognized by persons of skill in the art, the rate of material removal and the smoothness of the resulting surface is largely determined by the size and shape of the abrasives used during grinding. The relative material removal between the two different adhesive plates and the degree of subsurface damage caused by the grinding process can also be adjusted by varying the speed or direction of rotation of the two different plates or the carriers. For example, faster rotation of the coarse abrasive plate will allow for desired material removal from the bottom side of the wafer while the finer abrasive grinding process is being completed. For example, by adjusting the relative speeds of the coarse and fine abrasive grinding plates, the coarse abrasive could be used to remove, for example, 40 to 50 microns of material during the same time that the finer abrasive grinding plate is used to remove only 15 microns of material. In some prior art double-sided grinding or lapping machines, one plate is fixed while the other plate rotates to produce a relative velocity between the fixed and rotating plates. In these machines, the relative velocity between the rotating and non-rotating plates can also be adjusted to achieve the same desired material removal rates for the coarse and fine plates.

The coarse abrasive plate may be the lower or bottom plate and the finer abrasive may be the upper plate. In this embodiment, gravity will help prevent any loose abrasive particle or swarf from the coarse abrasive from marring or damaging the substrate surface processed with the finer abrasive. As will be recognized by persons of skill, the presence of the finer particulate will not negatively affect the surface finish on the coarser abrasive side of the substrate.

In particular embodiments, a grinding fluid (coolant) is circulated to remove particulate (swarf) from the surfaces of the abrasive plates during processing. The grinding fluid may be recirculated and, as a result, there is a possibility that abrasive particulate and swarf might be inadvertently introduced between the substrate body and the finer adhesive plate. If the grinding fluid is recirculated, the grinding fluid can be filtered to substantially reduce or prevent coarse abrasive particles or swarf from damaging the smoother surface of the substrate processed with the finer abrasive.

FIG. 4 schematically illustrates a filtration system that could be used to substantially reduce or prevent coarse abrasive material from inadvertently damaging the more polished substrate surface in accordance with particular embodiments. In the filtration system of FIG. 4, clean coolant is delivered using a coolant supply line 402 that can extend through the top grinding plate. Coolant flows during the entire grinding operation in particular embodiments. A suitable coolant flow rate will provide adequate lubrication to substantially reduce or prevent the substrates from being damaged by friction buildup and will flush away grinding debris. The coolant will flow (via gravity) down through the bottom grinding plate and out of the grinding machine through coolant return line 404. The coolant can then flow into coolant storage tank 406 for recirculation. Coolant in the storage tank can first undergo centrifuge filtration 407 to separate out swarf and grinding debris and then flow through a bag or cartridge filter 408. The bag or cartridge filter 408 will filter out any abrasive particles that are larger than the fine abrasive particles in order to substantially reduce or prevent coarse abrasive particle from marring or damaging the smoother upper surfaces of the substrates. The size of the final filtration can be determined by the particular application.

Applicants also note that the use of different abrasives on the upper and lower grinding plates may tend to increase the likelihood that the processed substrate will exhibit unacceptable warping or bowing. The abrasive grits used, along with the rotation speed and directions can be optimized to reduced any stress differential within the substrate body that could produce such changes in wafer shape.

Sapphire substrates produced using the methods described above are not only produced faster and at lower cost than using prior art methods, the finished substrates also have improved dimensional geometry over those produced by conventional processing. In particular aspect, a high surface area sapphire substrate produced in accordance with embodiments described herein includes a generally planar surface having an a-plane orientation, an r-plane orientation, an m-plane orientation, or a c-plane orientation, and which includes controlled dimensionality. As used herein, “x-plane orientation” denotes the substrates having major surfaces that extend generally along the crystallographic x-plane, typically with slight misorientation from the x-plane in accordance with particular substrate specifications, such as those dictated by the end-customer. Particular orientations include the r-plane and c-plane orientations, and certain embodiments utilize a c-plane orientation.

As noted above, the substrate may have a controlled dimensionality. One measure of controlled dimensionality is total thickness variation, including TTV (total thickness variation) or nTTV (normalized total thickness variation).

For example, in an embodiment, the TTV is generally around 3.00 microns, such as not greater than about 2.85 microns, or even not greater than about 2.75 microns. The foregoing TTV parameters are associated with large-sized wafers, and particularly large-sized wafers having controlled thickness. For example, embodiments may have a diameter not less than about 6.5 cm, and a thickness not greater than about 490 microns. In accordance with certain embodiments, the foregoing TTV parameters are associated with notably larger sized wafers, including those having diameters not less than 7.5 cm, not less than 9.0 cm, not less than 9.5 cm, or not less than 10.0 cm. Wafer size may also be specified in terms of surface area, and the foregoing TTV values may be associated with substrates having a surface area not less than about 40 cm², not less than about 70 cm², not less than about 80 cm², or even not less than about 115 cm². In addition, the thickness of the wafers may be further controlled to values not greater than about 500 microns, such as not greater than about 490 microns.

It is noted that the term “diameter” as used in connection with wafer, substrate, or boule size denotes the smallest circle within which the wafer, substrate, or boule fits. Accordingly, to the extent that such components have a flat or plurality of flats, such flats do not affect the diameter of the component.

Various embodiments have well controlled nTTV, such as not greater than about 0.037 μm/cm². Particular embodiments have even superior nTTV, such as not greater than 0.035 μm/cm², or even not greater than 0.032 μm/cm². Such controlled nTTV has been particularly achieved with large substrates, such as those having a diameter not less than about 9.0 cm, or even not less than about 10.0 cm. Wafer size may also be specified in terms of surface area, and the foregoing nTTV values may be associated with substrates having a surface area not less than about 90 cm², not less than about 100 cm², not less than about 115 cm³.

Referring to the total thickness variation values of the sapphire substrate, TTV is the absolute difference between the largest thickness and smallest thickness of the sapphire substrate (omitting an edge exclusion zone which typically includes a 3.0 mm ring extending from the wafer edge around the circumference of the wafer), and nTTV is that value (TTV) normalized to the surface area of the sapphire substrate. A method for measuring total thickness variation is given in ASTM standard F1530-02.

Generally, the nTTV value, as well as all other normalized characteristics disclosed herein, is normalized for a sapphire substrate having a generally planar surface and substantially circular perimeter which can include a flat for identifying the orientation of the substrate. In a particular embodiment, the sapphire substrate has a surface area of not less than about 25 cm², such as not less than about 30 cm², not less than 35 cm² or even not less than about 40 cm². Still, the substrate can have a greater surface area such that the generally planar surface has a surface area not less than about 50 cm², or still not less than about 60 cm², or not less than about 70 cm². The sapphire substrates may have a diameter greater than about 5.0 cm (2.0 inches), such as not less than about 6.0 cm (2.5 inches). However, generally the sapphire substrates have a diameter of 7.5 cm (3.0 inches) or greater, specifically including 10 cm (4.0 inches) wafers.

In further reference to characteristics of the sapphire substrate, in an embodiment, one generally planar surface of the sapphire substrate has a surface roughness Ra of not greater than about 100.0 Å, such as not greater than about 75.0 Å, or about 50.0 Å, or even not greater than about 30.0 Å. Even more superior surface roughness can be achieved, such as not greater than about 20.0 Å, such as not greater than about 10.0 Å, or not greater than about 5.0 Å. The other major surface of a sapphire substrate will have a much higher surface roughness since that second surface is only subjected to coarse grinding or lapping rather than any fine grinding or polishing. The second, coarser surface will preferably have a surface roughness of at least 7000 Å, at least 5000 Å, or at least 4000 Å.

The generally planar surface of the sapphire substrate processed in accordance with the methods described above can have superior flatness as well. The flatness of a surface is typically understood to be the maximum deviation of a surface from a best-fit reference plane (see ASTM F 1530-02). In this regard, normalized flatness is a measure of the flatness of the surface normalized by the surface area on the generally planar surface. In an embodiment, the normalized flatness (nFlatness) of the generally planar surface is greater than about 0.100 μm/cm², such as not greater than about 0.080 μm/cm², or even not greater than about 0.070 μm/cm². Still, the normalized flatness of the generally planar surface can be less, such as not greater than about 0.060 μm/cm², or not greater than about 0.050 μm/cm².

Sapphire substrates processed in accordance with methods provided herein can exhibit a reduced warping as characterized by normalized warp, hereinafter nWarp. The warp of a substrate is generally understood to be the deviation of the median surface of the substrate from a best-fit reference plane (see ASTM F 697-92(99)). In regards to the nWarp measurement, the warp is normalized to account for the surface area of the sapphire substrate. In an embodiment, the nWarp is not greater than about 0.190 μm/cm², such as not greater than about 0.170 μm/cm², or even not greater than about 0.150 μm/cm².

The generally planar surface can also exhibit reduced bow. As is typically understood, the bow of a surface is the absolute value measure of the concavity or deformation of the surface, or a portion of the surface, as measured from the substrate centerline independent of any thickness variation present. The generally planar surface of substrates processed in accordance with methods provided herein exhibit a reduced normalized bow (nBow) which is a bow measurement normalized to account for the surface area of the generally planar surface. As such, in one embodiment the nBow of the generally planar surface is not greater than about 0.100 μm/cm², such as not greater than about 0.080 μm/cm², or even not greater than about 0.070 μm/cm². In another embodiment, the nBow of the substrate is within a range of between about 0.030 μm/cm² and about 0.100 μm/cm², and particularly within a range of between about 0.040 μm/cm² and about 0.090 μm/cm².

Many different aspects and embodiments are possible. Some of those aspects and embodiments are described herein. After reading this specification, skilled artisans will appreciate that those aspects and embodiments are only illustrative and do not limit the scope of the present invention. Embodiments may be in accordance with any one or more of the items listed below.

Item 1. A method of machining a wafer having first and second opposing major surfaces, the method including grinding a first major surface of a wafer using a first fixed abrasive; and grinding a second major surface of the wafer using a second fixed abrasive, the second fixed abrasive having a grit size that is coarser than the grit size of the first fixed abrasive, wherein at least a portion of the grinding of the first and second major surfaces of the wafer occurs simultaneously.

Item 2. The method of Item 1, wherein the wafer is a sapphire substrate.

Item 3. The method of any one of Items 1 or 2, wherein the first fixed abrasive has a mean abrasive particle size of no more than 5 microns, no more than 20 microns, no more than 35 microns, or no more than 75 microns.

Item 4. The method of any one of the preceding Items, wherein the second fixed abrasive has a mean abrasive particle size of at least 60 microns, at least 80 microns, at least 100 microns, or at least 200 microns.

Item 4′. The method of any one of the preceding Items, wherein the difference between the average abrasive particle size in the upper fixed abrasive disk and the average abrasive particle size in the lower fixed abrasive disk is at least 20 microns, at least 50 microns, or at least 100 microns.

Item 5. The method of any one of the preceding Items, wherein grinding a first major surface of a wafer and grinding a second major surface of the wafer includes grinding the wafer between a first abrasive plate and a second abrasive plate, the second abrasive plate having a coarser abrasive than the first abrasive plate, wherein the first abrasive plate grinds the first major surface of the wafer and the second abrasive plate grinds the second major surface of the wafer.

Item 6. The method of Item 5, wherein the second abrasive plate is located underneath the first abrasive plate so that the second abrasive plate grinds the bottom surface of the wafer and the first abrasive plate grinds the top surface of the wafer.

Item 7. The method of any one of Items 1 to 4, wherein grinding a first major surface and grinding a second major surface of the wafer includes placing a wafer between first and second abrasive plates so that the top face of the wafer is in flat contact with the abrading surface of the first abrasive plate and the bottom face of the wafer is in flat contact with the abrading surface of the second abrasive plate; and rotating the abrasive plates, the wafer, or any combination thereof to grind the top and bottom faces of the wafer.

Item 8. The method of any one of Items 1 to 4, wherein grinding a first major surface and grinding a second major surface of the wafer includes placing at least one wafer into a circular carrier located between first and second abrasive plates, bringing the top face of the wafer into flat contact with the abrading surface of the first abrasive plate and the bottom face of the wafer into flat contact with the abrading surface of the second abrasive plate, rotating the abrasive plates, and rotating the carrier to rotate the wafer between the rotating abrasive plates.

Item 9. The method of Item 8, wherein a plurality of wafers are placed into the circular carrier.

Item 10. The method of any one of Items 8 or 9, wherein rotating the carrier includes causing the carrier to rotate about its own axis and around the central axis of the abrasive plates.

Item 11. The method of any one of Items 5 to 10 further including applying a predetermined pressure to the top and bottom surfaces of the wafer with the abrading surfaces of the abrasive plates while grinding.

Item 12. The method of any one of Items 5 to 11, wherein the relative material removal and the degree of any subsurface damage caused by grinding can be adjusted by varying the speed or direction of rotation of the wafer relative to at least one abrasive plate.

Item 13. The method of any one of Items 5 to 12, wherein the second abrasive plate removes 40 to 50 microns of material during the same time that the first abrasive plate removes 10 to 15 microns of material.

Item 14. The method of any one of Items 5 to 13, further including applying a grinding fluid to cool the grinding surfaces and to remove loose abrasive material or swarf.

Item 15. The method of Item 14, further including recirculating the grinding fluid after it has been used to cool the grinding surfaces and to remove loose abrasive material or swarf and filtering the used grinding fluid before it is reintroduced to prevent loose coarse abrasive particles in the recirculated grinding fluid from damaging the surface of the wafer during grinding.

Item 16. The method of any one of the preceding Items, wherein grinding the wafer with the second fixed abrasive removes 30 to 50 microns of material during the grinding process.

Item 17. The method of any one of the preceding Items, wherein grinding the wafer with the first fixed abrasive removes 10 to 15 microns of material during the grinding process.

Item 18. The method of any one of the preceding Items, wherein, when the grinding process is completed, the surface roughness on side of the wafer ground by the second fixed abrasive will be at least 4000 Å, at least 5000 Å, or at least 7000 Å.

Item 19. The method of any one of the preceding Items, wherein, when the grinding process is completed, the surface roughness on side of the wafer ground by the first fixed abrasive will be no more than 1000 Å, no more than 500 Å, or no more than 100 Å.

Item 20. The method of any one of the preceding Items, wherein the wafer includes a single crystal substrate.

Item 21. The method of any one of the preceding Items, wherein the wafer includes a polycrystalline material.

Item 22. The method of any one of the preceding Items, wherein the wafer includes sapphire, silicon carbide or gallium nitride.

Item 23. The method of any one of the preceding Items, wherein the wafer includes a glass, a ceramic, or a metallic compound.

Item 24. An apparatus for the double-sided grinding of a flat substrate, the apparatus including:

an upper and a lower grinding plate, the two grinding plates being coaxially mounted so that a substrate can be disposed between the two grinding plates and the two grinding plates being rotatable about their coaxial central axis by a grinding plate driving mechanism;

a substrate carrier disposed between the two grinding plates, the carrier including a carrier driving mechanism for rotating the carrier about its own central axis and about the coaxial central axis of the upper and lower grinding plates;

an upper fixed abrasive disk mounted to the inner surface of the upper grinding plate and a lower fixed abrasive disk mounted to the inner surface of the lower grinding plate, wherein the lower fixed abrasive disk has a coarser abrasive grit than the upper fixed abrasive disk so that double-sided substrate grinding of a substrate will remove material from the opposing substrate surfaces at different rates and so that double-sided substrate grinding will produce opposing substrate surfaces having different surface roughness.

Item 25. The apparatus of Item 24, wherein the substrate includes a single crystal substrate.

Item 26. The apparatus of Item 24, wherein the substrate includes a polycrystalline material.

Item 27. The apparatus of Item 24, wherein the substrate includes sapphire, silicon carbide or gallium nitride.

Item 28. The apparatus of Item 24, wherein the substrate includes a glass, a ceramic, or a metallic compound.

Item 29. Any one of Items 24 to 28, wherein the upper fixed abrasive disk, the lower fixed abrasive disk, or both include abrasive particles.

Item 30. The apparatus of Item 29, wherein the abrasive particles include crystalline materials or ceramic materials.

Item 31. The apparatus of Item 29, wherein the abrasive particles include alumina, silica, silicon carbide, zirconia-alumina, or any combination thereof.

Item 32. The apparatus of Item 29, wherein the abrasive particles include diamond, cubic boron nitride, or any combination thereof.

Item 33. The apparatus of any one of Items 29 to 32, wherein the difference between the average abrasive particle size in the upper fixed abrasive disk and the average abrasive particle size in the lower fixed abrasive disk is at least 20 microns, at least 50 microns, or at least 100 microns.

Item 34. The apparatus of any one of Items 29 to 33, wherein the abrasive particles are irregular in shape.

Item 35. The apparatus of any one of Items 29 to 34, wherein the abrasive particles are circular, square, or hexagonal in shape.

Item 36. The apparatus of any one of Items 29 to 35, wherein the upper fixed abrasive disk, the lower fixed abrasive disk, or both include a bonded fixed abrasive.

Item 37. The apparatus of Item 36, wherein the bonded fixed abrasive includes abrasive particles fixed in a matrix.

Item 38. The apparatus of Item 37, wherein the matrix includes a metal or metal alloy.

Item 39. The apparatus of Item 37, wherein the matrix includes iron, aluminum, titanium, bronze, nickel, silver, or any combination thereof.

Item 40. The apparatus of Item 36, wherein the bonded fixed abrasive includes abrasive particles fixed in a resin, vitreous, or metal matrix.

Item 41. The apparatus of Item 36 in which the bonded fixed abrasive includes abrasive particles bonded together in a resin, vitreous, or metal matrix to form the abrasive disk.

Item 42. A method of machining a sapphire substrate including grinding a first surface of a sapphire substrate having a diameter of not less than using a first fixed abrasive; and grinding a second surface of the sapphire substrate using a second fixed abrasive, the second fixed abrasive having a grit size that is different from the grit size of the first fixed abrasive, wherein at least a portion of the grinding of the first and second sides of the sapphire substrate occurs simultaneously.

Item 43. A method of machining a wafer having a first and second opposing major surfaces, the method including grinding a first major surface of a wafer using a first fixed abrasive; and grinding a second major surface of the wafer using a second fixed abrasive, the second fixed abrasive having a grit size that is coarser than the grit size of the first fixed abrasive, wherein at least a portion of the grinding of the first and second major surfaces of the wafer occurs simultaneously.

Item 44. A method of simultaneous double-side processing of a flat substrate, the method including:

placing a flat substrate between a first abrasive plate and a second abrasive plates, the first and second abrasive plates being coaxial and each having an abrading surface, the abrading surface of the second abrasive plate including abrasive particles having a coarser grit size than the abrasive particles on the abrading surface of the first abrasive plate;

bringing the abrading surface of the first abrasive plate into flat contact with the top surface of the substrate and the abrading surface of the second abrasive plate into flat contact with the bottom face of the substrate; and

rotating the first abrasive plates, the second abrasive plate, the substrate, or any combination thereof to abrade the top and bottom faces of the substrate, the coarser grit size of the second abrasive plate causing a greater rate of material removal and resulting in a rougher surface on the bottom face of the substrate as compared to the top face of the substrate.

Item 45. A method of removing material from a wafer by double-side grinding with planetary kinematics, the method including:

sandwiching the substrate between a first and a second bonded fixed abrasive plate, said first and second abrasive plates each having an inwardly facing abrading surface, the abrading surface of the first abrasive plate having a finer grit than the second abrasive plate, and the second abrasive plate having a coarser grit than the first abrasive plate;

rotating the first and second abrasive plates, the wafer, or any combination thereof to simultaneously remove material from both the top and bottom surfaces of the wafer, the coarser grit of the second abrasive plate causing a higher material removal rate than the first abrasive plate and the finer grit of the first abrasive plate resulting in a smoother wafer surface than the second abrasive plate.

Item 46. The method of any of Items 42, 43, and 45, wherein the first fixed abrasive has a mean abrasive particle size of no more than 5 microns, no more than 20 microns, no more than 35 microns, or no more than 75 microns.

Item 47. The method of any one of Items 42, 43, 45, and 46, wherein the second fixed abrasive has a mean abrasive particle size of at least 60 microns, at least 80 microns, at least 100 microns, or at least 200 microns.

Item 48. Any of the preceding Items, wherein grinding a first and second surface of a wafer or sapphire substrate includes grinding a sapphire substrate between a first abrasive plate and a second abrasive plate, the second abrasive plate having a coarser abrasive than the first abrasive plate.

Item 49. Any of the preceding Items, wherein grinding a first and second surface of a wafer or sapphire substrate includes grinding a sapphire substrate between a first abrasive plate and a second abrasive plate, the second abrasive plate having a coarser abrasive than the first abrasive plate, and the second abrasive plate being located underneath the first abrasive plate so that the second abrasive plate grinds the bottom surface of the wafer or sapphire substrate and the first abrasive plate grinds the top surface of the wafer or sapphire substrate.

Item 50. Any of the preceding Items, wherein grinding a first and second surface of a wafer or sapphire substrate includes:

placing a sapphire wafer between the first and second abrasive plates so that the top face of the sapphire wafer is in flat contact with the abrading surface of the first abrasive plate and the bottom face of the sapphire wafer is in flat contact with the abrading surface of the second abrasive plate; and

rotating the abrasive plates, the sapphire wafer, or any combination thereof to abrade the top and bottom faces of the sapphire wafer.

Item 52. Any of the preceding Items, wherein a plurality of wafers or sapphire substrates is loaded into a circular carrier which is located between the first abrasive plate and the second abrasive plate.

Item 52. Any of the preceding Items, wherein grinding a first and second surface of a wafer or sapphire substrate includes placing a sapphire wafer into a circular carrier located between the first and second abrasive plates; bringing the top face of the sapphire wafer into flat contact with the abrading surface of the first abrasive plate and the bottom face of the sapphire wafer into flat contact with the abrading surface of the second abrasive plate; rotating the abrasive plates; and rotating the carrier to rotate the sapphire substrate between the rotating abrasive plates.

Item 53. Any of the preceding Items, wherein the substrates to be processed are held in a carrier that is disposed between the abrading surfaces of the two abrasive plates, and, wherein grinding a first and second surface of a wafer or sapphire substrate includes causing the carrier to rotate about its own axis and around the central axis of the abrasive plates.

Item 54. Any of the preceding Items, further including applying a predetermined inward pressure to the top and bottom surfaces of the wafer or sapphire substrate with the abrading surfaces of the abrasive plates while the carrier, abrasive plates, or any combination thereof are rotated.

Item 55. Any of the preceding Items, wherein the relative material removal between the two different adhesive plates and the degree of subsurface damage caused by the grinding process can also be adjusted by varying the speed or direction of rotation of the two different plates or the carriers.

Item 56. Any of the preceding Items, wherein the second abrasive plate removes 40 to 50 microns of material during the same time that the first abrasive plate removes 10 to 15 microns of material.

Item 57. Any of the preceding Items, further including applying a grinding fluid to the abrading surfaces of the first and second abrasive plates to cool the grinding surfaces and to remove loose abrasive material or swarf.

Item 58. Item 57, further including recirculating the grinding fluid after it has been used to cool the grinding surfaces and to remove loose abrasive material or swarf and filtering the used grinding fluid before it is reintroduced to the first abrasive plate to prevent coarse abrasive particles from the second abrasive plate from damaging the surface of the wafer or sapphire substrate being ground to a smoother finish by the first abrasive plate.

Item 59. An apparatus for the double-sided grinding of a flat substrate, the apparatus including:

an upper and a lower grinding plate, the two plates being coaxially mounted so that a substrate can be mounted between the two plates and the two plates being rotatable by a driving mechanism;

a carrier for holding a substrate disposed between the two plates, the carrier including a driving mechanism for rotating the carrier about its own central axis and about the coaxial central axis of the grinding plates;

an upper fixed abrasive plate mounted to the inner surface of the upper grinding plate and a lower fixed abrasive plate mounted to the inner surface of the lower grinding plate, wherein the lower fixed abrasive plate has a coarser abrasive grit than the upper fixed abrasive plate so that double-sided substrate grinding of a substrate will remove material from the opposing substrate surfaces at a different rate and so that double-sided substrate grinding will produce opposing substrate surfaces having different surface roughness.

Item 60. Any of the preceding Items, wherein the substrate includes a single crystal substrate.

Item 61. Any of the preceding Items, wherein the substrate includes a polycrystalline material.

Item 62. Any of the preceding Items, wherein the substrate includes sapphire, silicon carbide or gallium nitride.

Item 63. Any of the preceding Items, wherein the substrate includes a glass, a ceramic, or a metallic compound.

Item 64. Any of the preceding Items, wherein the first and second abrasives include abrasive particles.

Item 65. Item 64, wherein the abrasive particles include crystalline materials or ceramic materials.

Item 66. Item 64, wherein the abrasive particles include alumina, silica, silicon carbide, zirconia-alumina, or any combination thereof.

Item 67. Item 64, wherein the abrasive particles include diamond, cubic boron nitride, or any combination thereof.

Item 68. Item 64, wherein the difference between the average abrasive particle size in the first abrasive and the average abrasive particle size in the second abrasive is at least 20 microns, at least 50 microns, or at least 100 microns.

Item 69. Item 64, wherein the abrasive particles are irregular in shape.

Item 70. Item 64, wherein the abrasive particles are circular, square, or hexagonal.

Item 71. Any of the preceding Items, wherein at least one of the first and second abrasives includes a coated fixed abrasive.

Item 72. Any of the preceding Items, wherein the first and second abrasives include bonded fixed abrasives.

Item 73. Item 72, wherein the bonded fixed abrasives include abrasive particles fixed in a matrix.

Item 74. Item 72, wherein the bonded fixed abrasives include abrasive particles fixed in a resin, vitreous or metal matrix.

Item 75. Item 72, wherein the bonded fixed abrasives include abrasive particles bonded together in a resin, vitreous or metal matrix to form a rigid adhesive plate used for the microgrinding process.

Item 76. Any of the preceding Items, wherein the first and second abrasives include bonded fixed abrasives that include abrasive particles fixed in a matrix.

Item 77. Item 73, wherein the matrix includes a metal or metal alloy.

Item 78. Item 73, wherein the matrix includes iron, aluminum, titanium, bronze, nickel, silver,

Item 79. Any of the preceding Items, wherein the coarse grinding by the second abrasive plate removes 30 to 50 microns of material during the grinding process.

Item 80. Any of the preceding Items, wherein the fine grinding by the first abrasive plate removes 10 to 15 microns of material during the grinding process.

Item 81. Any of the preceding Items, wherein when the grinding process is completed, the surface roughness on the coarse abrasive side of the wafer or sapphire substrate will be at least 4000 Å, at least 5000 Å, or at least 7000 Å.

Item 82. Any of the preceding Items, wherein when the grinding process is completed, the surface roughness on the fine abrasive side will be no more than 1000 Å, no more than 500 Å, or no more than 100 Å.

Item 83. A finished sapphire substrate having a first side with a surface roughness of no more than 1000 Å and a second side with a surface roughness of at least 4000 Å.

Item 84. A finished sapphire substrate made using the method of any of the preceding Items.

By way of example, c-plane sapphire wafers having diameters of 4 inches could be processed in accordance with embodiments by applying the processing parameters described below.

Processing initiates with a boule or ingot that is sectioned or sliced, as described above. The boule is typically sectioned using a wire sawing technique. The wire sawing process can last several hours, usually within a range of between about 4 to 8 hours. It will be appreciated that the duration of the wire sawing process is at least partially dependent upon the diameter of the boule being sectioned and thus may last longer than 8 hours.

After wire sawing, the wafers have an average thickness of about 1.0 mm or less. Generally, the wafers have an average surface roughness (Ra) of less than about 1.0 micron, an average total thickness variation of about 30 microns, and an average bow of about 30 microns.

After wire sawing the boule to produce wafers, the wafers are subjected to a grinding process in accordance with embodiments as described herein. The wafers can be loaded into a double-sided micro-grinding machine such as a Peter Wolters AC 1000 or a PR Hoffman RC 5400. The bottom grinding plate can use a coarse vitrified grinding wheel having an average grit size within a range of about 80 to 200 microns. The coarse grinding plate will be rotated at approximately 60 to 500 rpm

The top grinding plate preferably uses a finer vitrified grinding wheel having an average grit size within a range of about 10 to 80 microns. The fine grinding plate preferably will be rotated at a slower speed that the coarse plate so that substrate material will be preferentially removed from the bottom surfaces of the substrates.

Any typical synthetic grinding fluid can be used as the coolant/grinding fluid.

In a particular embodiment, the process parameters above should result in a material removal rate (MRR) of approximately 5 to 10 μm/min for the coarse abrasive plate and a MRR of 1 to 5 μm/min for the fine abrasive plate. After grinding is complete, the sapphire substrates will preferably be around 1 mm thick. The surface roughness on the fine abrasive side will be around 0.1 μm (1000 Å), but may be as low as 500 Å. The surface roughness on the coarse abrasive side will be around 4000 Å, but may be as high as 7000 Å or more for some applications.

Once the grinding step is complete, the sapphire substrates may be further polished on the fine abrasive side to bring the surface roughness down to a mirror finish of 10 to 400 Å using conventional polishing methods.

Although much of the previous discussion is directed at sapphire wafers, embodiments described herein can be applied to any substrate production process that utilizes a coarse lapping or microgrinding, followed by a second finer polishing step to improve the surface finish or reduce surface damage that is needed on only one side of the substrate. For example, embodiments of the invention can be applied to the finishing (production) of oriented single crystal bodies, including sapphire and silicon carbide, other polycrystalline materials, ceramics, glasses, metals, plastics, etc. Furthermore, embodiments of the invention can be applied to any substrate or part that is currently processed in a grinding operation or fixed abrasive operation known as “microgrinding”, “grinding with lapping kinematics”, “grinding with planetary kinematics” or “fixed abrasive lapping” to produce a desired geometry and surface finish.

As used herein, the terms “wafer” and “substrate” are used herein synonymously to refer to sectioned sapphire material that is being formed or processed, to be used as a substrate for epitaxial growth of semiconductor layers thereon, such as to form an optoelectronic device. Oftentimes it is common to refer to an unfinished sapphire piece as a wafer and a finished sapphire piece as a substrate, however, as used herein, these terms do not necessarily imply this distinction.

The invention described herein has broad applicability and can provide many benefits as described and shown in the examples above. The embodiments will vary greatly depending upon the specific application, and not every embodiment will provide all of the benefits and meet all of the objectives that are achievable by the invention. Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed.

In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention. After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.

As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). Also, the use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized in accordance with the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method of machining a wafer having first and second opposing major surfaces, the method comprising: grinding a first major surface of a wafer using a first fixed abrasive; and grinding a second major surface of the wafer using a second fixed abrasive, the second fixed abrasive having a grit size that is coarser than the grit size of the first fixed abrasive, wherein at least a portion of the grinding of the first and second major surfaces of the wafer occurs simultaneously.
 2. The method of claim 1, wherein the wafer is a sapphire substrate.
 3. The method of claim 1, wherein the first fixed abrasive has a mean abrasive particle size of no more than 5 microns, no more than 20 microns, no more than 35 microns, or no more than 75 microns.
 4. The method of claim 1, wherein the second fixed abrasive has a mean abrasive particle size of at least 60 microns, at least 80 microns, at least 100 microns, or at least 200 microns.
 5. The method of claim 1, wherein the difference between the average abrasive particle size in the upper fixed abrasive disk and the average abrasive particle size in the lower fixed abrasive disk is at least 20 microns, at least 50 microns, or at least 100 microns.
 6. The method of claim 1, wherein grinding a first major surface of a wafer and grinding a second major surface of the wafer comprises grinding the wafer between a first abrasive plate and a second abrasive plate, the second abrasive plate having a coarser abrasive than the first abrasive plate, wherein the first abrasive plate grinds the first major surface of the wafer and the second abrasive plate grinds the second major surface of the wafer.
 7. The method of claim 6, wherein the second abrasive plate is located underneath the first abrasive plate so that the second abrasive plate grinds the bottom surface of the wafer and the first abrasive plate grinds the top surface of the wafer.
 8. The method of claim 6, further comprising applying a grinding fluid to cool the grinding surfaces and to remove loose abrasive material or swarf.
 9. The method of claim 8, further comprising recirculating the grinding fluid after it has been used to cool the grinding surfaces and to remove loose abrasive material or swarf and filtering the used grinding fluid before it is reintroduced to prevent loose coarse abrasive particles in the recirculated grinding fluid from damaging the surface of the wafer during grinding.
 10. The method of claim 1, wherein grinding a first major surface and grinding a second major surface of the wafer comprises: placing a wafer between first and second abrasive plates so that the top face of the wafer is in flat contact with the abrading surface of the first abrasive plate and the bottom face of the wafer is in flat contact with the abrading surface of the second abrasive plate; and rotating the abrasive plates, the wafer, or any combination thereof to grind the top and bottom faces of the wafer.
 11. The method of claim 1, wherein grinding a first major surface and grinding a second major surface of the wafer comprises: placing at least one wafer into a circular carrier located between first and second abrasive plates; bringing the top face of the wafer into flat contact with the abrading surface of the first abrasive plate and the bottom face of the wafer into flat contact with the abrading surface of the second abrasive plate; rotating the abrasive plates; and rotating the carrier to rotate the wafer between the rotating abrasive plates.
 12. The method of claim 1, wherein grinding the wafer with the second fixed abrasive removes 30 to 50 microns of material during the grinding process.
 13. The method of claim 1, wherein grinding the wafer with the first fixed abrasive removes 10 to 15 microns of material during the grinding process.
 14. The method of claim 1, wherein, when the grinding process is completed, the surface roughness on side of the wafer ground by the second fixed abrasive is at least 4000 Å, at least 5000 Å, or at least 7000 Å.
 15. The method of claim 1, wherein, when the grinding process is completed, the surface roughness on side of the wafer ground by the first fixed abrasive is no more than 1000 Å, no more than 500 Å, or no more than 100 Å.
 16. The method of claim 1, wherein the wafer comprises a single crystal substrate.
 17. The method of claim 1, wherein the wafer comprises a polycrystalline material.
 18. An apparatus for the double-sided grinding of a flat substrate, the apparatus comprising: an upper and a lower grinding plate, the two grinding plates being coaxially mounted so that a substrate can be disposed between the two grinding plates and the two grinding plates being rotatable about their coaxial central axis by a grinding plate driving mechanism; a substrate carrier disposed between the two grinding plates, the carrier including a carrier driving mechanism for rotating the carrier about its own central axis and about the coaxial central axis of the upper and lower grinding plates; and an upper fixed abrasive disk mounted to the inner surface of the upper grinding plate and a lower fixed abrasive disk mounted to the inner surface of the lower grinding plate, wherein the lower fixed abrasive disk has a coarser abrasive grit than the upper fixed abrasive disk so that double-sided substrate grinding of a substrate removes material from the opposing substrate surfaces at different rates and so that double-sided substrate grinding produces opposing substrate surfaces having different surface roughness.
 19. The apparatus of claim 18, wherein the difference between the average abrasive particle size in the upper fixed abrasive disk and the average abrasive particle size in the lower fixed abrasive disk is at least 20 microns, at least 50 microns, or at least 100 microns.
 20. The apparatus of claim 18, wherein the upper fixed abrasive disk, the lower fixed abrasive disk, or both comprise a bonded fixed abrasive. 